The human genome is littered with sequences derived from transposable elements from the Hsmar1 transposon, but there is only one intact copy of the Hsmar1 transposase gene termed Metnase (also known as SETMAR) that exists within a chimeric SET-transposase fusion protein. Although Metnase retains most of the transposase activities, it has evolved as a double-strand break (DSB) repair protein in anthropoid primates. Metnase is localized on chromosome 3p26, a region of frequent abnormalities in various cancers and is highly expressed in most tissues and cell lines. Mutations in Metnase that cause early termination were found in many transformed cell lines, although clinical relevance of these mutations has not been established. Our long-term goal is to understand how a protein with transposase activity in humans promotes DSB repair and chromosome decatenation, and what role the SET domain may play. Given that Metnase requires both the SET and transposase domains for its function(s) in DSB repair, we hypothesize that the acquisition of new functions may have resulted from a chimeric fusion between transposase and the SET domains. In this study we proposed three specific aims to elucidate the mechanism of this human SET- transposase protein in DSB repair and chromosome decatenation. Aim 1: Determine the mechanism by which Metnase is localized to DSB sites. We will identify the region within the SET domain of Metnase crucial for localization to DSB sites following IR treatment. We will also investigate whether the interaction of Metnase with Pso4, a Metnase binding partner that plays a crucial role in Metnase localization at DSB sites, is crucial for Metnase localization at DSB sites. Finally, we will examine whether Metnase directly interacts with the Ku70/80 complex. If so, a mutant Metnase defective in interaction with Ku complex will be generated, and we will determine how this mutant differs from wt-Metnase in their localization at DSB sites. Aim 2: Determine the role(s) of Metnase's biochemical activities in DNA end joining and chromosome decatenation. Metnase not only possesses a structure-specific endonuclease and HLMT activities, but also interacts with Lig4, Pso4, and Topo II1, all of which could play role(s) in NHEJ repair and/or chromosome decatenation. We will examine how Metnse's biochemical activities are involved in DNA end joining and chromosome decatenation. First, we will substitute key amino acids within the catalytic site identified from the crystal structure of Metnase transposase domain and examine the mutants for DNA cleavage, DNA end processing, and end joining activities. Secondly, we will examine how Metnase's interaction with Lig4 affects recruitment of Lig4-XRCC4 to DSB sites and DNA end joining. Thirdly, we will examine how Metnase binding partner (Pso4) and its interaction with Metnase influence DNA end joining. Fourth, we will examine whether Metnase mutant(s) lacking HLMT and/or auto-methylation activity support stimulation of DSB repair. Finally, Metnase mutants lacking its biochemical activities will be examined for promotion of chromosome decatenation activity.